1. Field of the Invention
The present invention relates to an encoder apparatus, or more specifically an encoder that is included in the encoder apparatus as one of its components, wherein the encoder apparatus may be mounted on a rotational member in an automotive vehicle (such as between the outer and inner races of the wheel bearing unit on the driving shaft or driven shaft) for detecting the number of revolutions of the rotational member.
2. Description of the Prior Art
One example of the conventional encoder apparatus that may be mounted on a rotational member in the automotive vehicle by pressing the encoder apparatus into the rotational member for detecting the number of revolutions of the rotational member is disclosed in Japanese patent application as now published under No. 62 (1987)-25267, for example, wherein the encoder apparatus includes an encoder in the form of a magnetic signal generator ring.
As described in the above document, the magnetic signal generator ring is based on a synthetic resin material that is mechanically strong enough to avoid any possible damages that might occur on the ring as it is pressed into the rotational member, and includes an annular synthetic resin magnet that is buried around the outer peripheral surface of the ring. The synthetic resin magnet takes the form of a multipole magnet having S polarities and N polarities magnetized alternately at equal intervals in the circumferential direction.
As the encoder apparatus is mounted on the rotational member in the manner described above, the encoder in the encoder apparatus may be placed to face opposite the sensor that is located adjacently to the encoder outside it.
As the rotational member on which the encoder apparatus is mounted is thus rotating at the number of revolutions that is changing every moment, the encoder may magnetically produce pulses each of which represent the respective ever-changing number of revolutions and the sensor may detect the ever-changing number of revolutions by responding to each of the pulses.
In the conventional encoder apparatus described above, however, there is a risk that some extraneous matter such as stones might enter the area between the encoder in the encoder apparatus and the sensor located to face opposite the encoder outside it. This extraneous matter such as stones might become engaged between the encoder and the sensor, and thus cause damage to the encoder.
The side of the encoder facing opposite the sensor is magnetized as described above, acting as the magnetized surface having alternate N polarities and S polarities at equal intervals. If this magnetized surface is damaged by the extraneous matter such as stones which enter the area between the encoder and sensor, the sensor would not be able to detect the number of revolutions correctly because the encoder would fail to function properly. Thus, this presents a serious disadvantage.
Another example of the conventional encoder apparatus that includes an encoder known as the annular encoder is disclosed in Japanese patent application as now published under No. 2001-241435, wherein the encoder apparatus may be mounted on a rotational member, such as between the inner and outer races of the bearing unit rotating relative to each other, so that it can detect the number of revolutions. In this conventional encoder apparatus, the encoder is covered by a nonmagnetic cover on the side thereof facing opposite the sensor in order to avoid damaging the encoder as described above. Other examples of the encoder apparatus are disclosed in Japanese patent applications as now published under No. H5 (1993)-249126, No. H11 (1999)-303879, and No. 2002-286739, respectively.
In recent years, the encoder apparatus that may be mounted on a rotational member on the automotive vehicle by pressing the encoder apparatus into the rotational member for detecting the ever-changing number of revolutions for the rotational member has more often been used with the FF (front engine, front drive) vehicle in particular, in which the encoder apparatus is mounted on the drive shaft and the like, and is used under the more severe running or ambient conditions.